As the new volunteers took to the streets on the eve of key congressional testimony by Army Gen. David Petraeus, the top U.S. military commander in Iraq, Iraqi security forces backed by U.S. forces clashed again with the Mahdi Army in the Shiite slum of Sadr City, and U.S. aircraft attacked militants.

In the heavily fortified Green Zone, where U.S. and Iraqi officials live and work, rocket attacks killed two U.S. soldiers and injured 17, at least five of them seriously. Rocket attacks on a U.S. base in southeast Baghdad killed another American soldier and wounded 14.

In some Mahdi Army strongholds, new government-backed militiamen were manning checkpoints with Iraqi National Police and others were piling up sandbags in anticipation of possible attacks.

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Thaer Mahdi, 18, sat at a checkpoint in the southwest Baghdad neighborhood of al Alam, which Sadr's forces overran last week in intense fighting with Iraqi government and U.S. forces.

"Yesterday we appeared in the streets of this area," Mahdi said, sitting on a street where the militia last week captured a police station and checkpoints and seized police weapons and vehicles. "We have orders to deploy here . . . . I want to tell you that (the militias) cannot overrun us, and we are not afraid to face them."

It was unclear, however, who gave the orders to Mahdi and others, and whether anyone is paying the Shiite volunteers.

The U.S. has been recruiting, training and paying local Sunni forces to fight Sunni militants, but U.S. officials Sunday said the American military had no new contracts with Shiite militia members. Phone calls to Iraqi government officials went unanswered.

Residents in two Mahdi Army-dominated neighborhoods in west Baghdad, Bayaa and Amil, however, said they expected a U.S.-backed government militia to be stood up there soon.

A senior Iraqi official said that U.S.-backed Iraqi Prime Minister Nouri al Maliki is now trapped in his battle with Sadr's militia. A million-man march that Sadr has called for April 9 to protest the U.S. occupation will likely bring more fighting.

"We are expecting more trouble . . . . This is not a resolution, and I think we are going to see an eruption," said the official, who asked not to be named due to the sensitivity of the subject. "It was untimely, and (Maliki) went there unprepared to take on the militia. He didn't recognize the scale of this movement, and he went there and it blew up in our faces in Baghdad."

The new Shiite neighborhood militias may be intended to compensate for the Iraqi Security Forces' poor performance in last week's offensive against the Mahdi Army in the southern port city of Basra. Hundreds deserted, and in Baghdad Shiite militants overran police stations and Security Forces checkpoints.

Gen. Jeffrey Hammond, the senior U.S. military commander in Baghdad, said that at the height of last week's battle there were more than 100 attacks a day in Baghdad, compared to 13 the week before.

"These men are supporting councils to support the troops," said 1st Lt. Kareem Mohammed of the National Police. "The people will not reject them because they are from the neighborhood and they have the agreement from the authorities to do what they are doing."

Hammond and Gen. Aboud Qanbar, the Iraqi commander of Baghdad operations, said the Iraqi forces are prepared for a larger fight.

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"The glass is half-full," Hammond said. "This is the first time they've been tested, and they are doing well."

The often-Orwellian battle of words in Iraq, however, is at least as confusing as the fighting in the streets.

Maliki promised to liberate the Mahdi Army strongholds of Sadr City and Shoala, then said government forces wouldn't raid Sadr strongholds until the ultimatum to the renegade Shiite militias runs out on April 8.

Sadr asked his militia to stand down after an Iranian-brokered ceasefire, but days later issued a statement saying his forces were being forced to choose between "drawing swords and degradation".

Sadr's followers and analysts see the battle as a Shiite fight for power in the south.

Days before Maliki launched the Basra offensive, a provincial powers law was rammed through Iraq's presidency council during a visit to Baghdad by Vice President Dick Cheney.

"They refused to accept the decision to accomplish those elections," said Salah al Obaidi, Sadr's spokesman. "But later on, after the visit of Dick Cheney, they were forced to accept the elections. We think that this crisis was the substitution."

Elections, however, would undercut the power of the Islamic Supreme Council of Iraq, which is more powerful but less popular among ordinary Iraqis than Sadr's movement is. Maliki closely allied himself with the Shiite movement after he turned his back on the Sadrists, who'd help get him elected.

When the battle began, Sadr showed that contrary to claims by the U.S. military, his militia was organized and ready to fight.

"The Mahdi Army is an organized militia," the senior Iraqi official said. "It's not a splinter group or disorganized group, and they all follow the sayings of their leader."

"We call them special groups. They are not just purely criminals. They are obviously organized," said Lt. Col. Dan Barnett, the head of the 1st Squadron, 2nd Stryker Cavalry Regiment, based in Vilseck, Germany and operating in Sadr City. "They have a command and control structure and a plan in place."

It was still unclear to Barnett if the cell leaders they'd identified followed Sadr.

"What we want to see is a de-escalation of the situation. We want everyone to go back to their corners," he said. "We don't want to see a big slugfest. That's not in anyone's interest."